Really, really different cultures!
There are a lot of similarities - like language characters like chan mentioned, even though those characters often have different meanings - but they definitely have very distinct characteristics.
Think about England and France... some similar language backgrounds, some ethnic intermingling over the centuries, many similar cultural influences, but definitely two distinct cultures.
In the case of Japan and China, the difference is even greater, since they culturally diverged a lot longer ago than England/France (i.e. Norman invasion 1066).
Japan - Shinto
China - Confucionism/Daoism
Japan - rice and fish
China - Much more diverse, depending a lot more on region
Japan - very modern with clean, high density housing. Used technological innovation to rise to one of the top economies of the 80's-90's. Currently in a state of stagnant modernity.
China - A huuuuge disparity between the large rural areas where running water can still be a luxury, the large urban cities, and the fast-blooming industrial 'factory cities.' Cheap manufacturing, roughshod modernization efforts in some locations, and a big trade surplus mark the country's current condition.